Field
Aspects of the present invention generally relate to a belt conveying device used in an image forming apparatus, such as a copier, a printer, and a facsimile machine of an electrophotographic system or an electrostatic recording system, and relates also to an image forming apparatus provided with the belt conveying device.
Description of the Related Art
In an image forming apparatus of an electrophotographic system or an electrostatic recording system, a belt conveying device provided with an endless belt (hereafter, also referred to as a “belt”) trained around a plurality of tension rollers is used. The belt is used as a conveyance member which bears and conveys a toner image, and bears and conveys a recording material on which the toner image is formed. The conveyance member which bears and conveys the toner image may be, for example, a belt-shaped electrophotographic photoconductor (a photoconductor belt), and an intermediate transfer member (an intermediate transfer belt) which bears and conveys the toner image transferred from the photoconductor to be transferred to the recording material. The conveyance member which bears and conveys the recording material on which the toner image is formed may be, for example, a recording material bearing member (a conveying belt) which bears and conveys the recording material on which the toner image is transferred from the photoconductor.
A belt trained around a plurality of tension rollers and driven to rotate (i.e., conveyed) generally has a problem of “belt deviation (i.e., meandering)” which is a phenomenon that the belt deviates toward either one end portion in the width direction when driven to rotate. The belt deviation occurs because of diametral accuracy of each tension roller, alignment accuracy among the tension rollers, and other factors.
To avoid belt deviation, active steering control has been proposed (see Japanese Patent Laid-Open No. 2002-2999 and No. 2010-223981). In active steering control, deviation of the belt from a predetermined position in the width direction is detected and at least one tension roller (i.e., a steering roller) is tilted to other tension rollers, whereby the belt is moved in the direction opposite to that of the deviation.
Another image forming apparatus is capable of forming an image while switching a conveyance speed of a belt among a plurality of speeds depending on, for example, the type of a recording material used for the output of the image. Setting a tilting speed of a steering roller to be lower as the conveyance speed of the belt becomes smaller so that steering control does not become unstable even when the belt is conveyed at a plurality of conveyance speeds is proposed (see Japanese Patent Laid-Open No. 2000-305415).
If the steering roller is tilted in accordance with the deviation of the belt in the width direction, however, the belt is twisted and the conveyance condition (e.g., speed) of the belt changes transitionally. The influence of the change in the conveyance condition of the belt tends to become significant especially when the belt is used as, for example, an intermediate transfer belt to which toner images are transferred from a plurality of image bearing members in a tandem image forming apparatus. This is because the change in the conveyance condition of the belt on a surface to which the toner images are transferred from a plurality of image bearing members (i.e., an image transfer surface) causes misalignment in positions at which the toner images of a plurality of colors overlap one another, and causes “color misalignment” which may impair quality of an output image. If a moving speed of the belt in the width direction to correct the deviation of the belt position in the width direction is high, color misalignment may easily be caused especially in the width direction of the belt.
In order to reduce the change in the conveyance condition of the belt to reduce the influence on the image quality, it is desirable to perform a tilting operation of the steering roller sufficiently gently. If the conveyance speed of the belt is low, it is more desirable to perform the tilting operation of the steering roller relatively more gently.
However, as an operation of a mechanism system which causes the steering roller to be tilted becomes more quasi-static, an influence of friction becomes larger, and the tilting operation may become unstable due to backlash and stick slip of mechanical components. That is, there is a limit in making the tilting operation itself of the steering roller gentle (i.e., making the tilting speed itself lower).